Life in the rural Ardèche quickly bored him out of his wits, however, and so he left for Paris. During his stint with the Navy he had made some acquaintances that were part of the Parisian crime scene. In Paris he sought them out and quickly became part of the Paris underworld, doing his part working out of the Montmartredistrict. Henri slowly becomes a known crime figure and during this period of stealing, breaking into safes and, as some say, pimping, he acquires his nickname Papillon2.

Right after I wrote the above wu I saw the movie for the first time.
The movie is definitely worth watching, but read the book first.
It's definitely the better of the two.

Sources:
http://gofree.indigo.ie/~james079/pape.html
http://www.sinclairos.freeserve.co.uk/Papillon/PAPILLON2.htm
http://www.jecs.nl/~papillon/1 My source states 'Saint-Etienne-de Lugares', but there is no place in the Ardèche to be found with that name.2 Some sources say the nickname was pinned to him earlier, during his stint in the Navy, and was inspired by the tattoo of a butterfly on his chest. Papillon is, of course, the French word for butterfly.